1. Field of the Invention
This invention relates to nuclear reactor fuel element assemblies and, in particular, to a fuel element assembly which utilizes a grid plate arrangement for locating and supporting fuel elements in the form of pins, rods or the like.
2. Summary of the Prior Art
In heterogeneous nuclear reactors, nuclear fuel is separated from the moderator and arranged in discrete bodies known as fuel elements. Fuel elements typically utilized in heterogeneous reactors consist of thin-walled, elongated, slender tubes or rods which clad the nuclear fuel contained within the element in order to prevent corrosion of the fuel and the release of fission products into the coolant, and are known in the art as "fuel pins" or "fuel rods". Aluminum or its alloys, stainless steel and zirconium alloys are common cladding materials. Such fuel pins are generally arranged in a carefully designed pattern to form an array which comprises the reactor core that provides the concentration of fissionable material needed to sustain a continuous sequence of fission reactions. In a heterogeneous reactor the fuel pins in the core become depleted at different rates, those in the center usually being subjected to a higher neutron flux and thus becoming depleted before those near the outside of the core where a lower neutron flux prevails. Consequently, all of the fuel elements are not normally replaced at one time but rather in stages. Furthermore, at each refueling, partially depleted elements may be relocated in order to optimize core performance and extend the time between refueling outages. It is advantageous, therefore, to group the fuel elements into movable units, known as fuel assemblies, which may contain hundreds of fuel pins. A fuel assembly is typically arranged in juxtaposition with similar assemblies in the core of a pressurized water reactor. In a boiling water reactor, each fuel assembly is typically encased in a square flow channel, commonly called a "can", which is juxtaposed with similar cans occupying the core. Movement of the fuel elements as fuel assemblies during charging and discharging of a reactor core expedites core reloading operations, thereby increasing the overall availability of the reactor and generally enhancing the economics of nuclear reactor use for functions such as power generation.
The design of a fuel assembly requires careful analysis to assure the maintenance of the assembly's geometrical integrity during all phases of reactor operation. Heat generated within the fuel pin is often removed by a fluid coolant which flows through the reactor core generally in a direction which is parallel to the longitudinal axes of the fuel pins. The fluid velocity and flow rate may be very high in order to remove the large quantity of heat generated. The surface area of the individual fuel pins, therefore, must be as fully exposed to the flowing fluid as possible in order to promote heat transfer to the coolant and to prevent the development of hot spots on the fuel element due to poor coolant flow conditions. Moreover, the elongated slender fuel pins may be subjected to harmful vibrations induced by the coolant flow or other sources.
Thus, it is desirable to arrange fuel elements in an assembly wherein the elements are spaced in a geometry conducive to proper reactor physics while satisfying a number of conflicting needs, viz., the need to minimize structural restraints in order to promote heat transfer from the fuel pins to the coolant, the need to provide structural support to a large number of fuel pins subjected to thermal, hydraulic and vibratory forces and the like, the need to minimize hydraulic pressure losses, and the need to minimize the presence of material capable of parasitic absorption of neutrons. Some fuel assemblies of the prior art have utilized a grid of plates to space and support the fuel pins. Usually, these grids comprise a cellular structure, commonly characterized as the egg crate design, that is formed through the mutually perpendicular intersections of a group of interlocking metal plates. Bosses, dimples, bowed members and the like protrude from the surface of the portions of these interlocking plates that form the individual cell walls. A fuel pin is inserted into each cell formed in the grid structure. The protrusions engage the outer surface of the fuel pin within a particular cell both restraining and locating the pin.
Two types of protrusions are commonly employed. One type of grid plate protrusion is very resilient being essentially spring mounted. The resilient character of these protrusions permits their deflection so that the fuel pins can be inserted into the grid structure with relative ease. Upon removal of the deflecting means the resilient protrusion springs back into position in the cell thus receiving the fuel pin. The other type of grid plate protrusion is a very stiff, rigid member which essentially eliminates relative movement between the fuel pins and the protrusions.
Problems have been experienced in grid designs in which either resilient or rigid protrusions alone have been used. Construction of a grid with cells containing a totality of resilient protrusions is difficult. Use of a two-tier arrangement of grids to overcome such difficulties results in the introduction of additional material capable of parasitic absorption of neutrons while increasing costs and complicating fabrication of the fuel assembly. During reactor operation the flexibility of the resilient protrusions permits relative movement at the protrusion to fuel pin contact point. This motion produces an undesirable wearing or "fretting" of the pin that weakens the cladding and can cause its failure. Use of a totality of the rigid type of protrusions, on the other hand, leads to other difficulties. For example, it is difficult to insert a fuel pin through a cell containing a totality of the unyielding rigid protrusions without galling, abrasion, gouging or like damage to the cladding.
A grid plate design which utilizes a combination of resilient protrusions and rigid protrusions within a cell can overcome these problems. Deflection of the resilient protrusions allows fuel pin insertion without damage. After removal of the deflecting means, the resilient protrusions spring into position causing the fuel pins to be secured at the contact points of both the resilient and rigid protrusions. It is evident that in each cell a resilient protrusion should be located on the plate wall opposite a plate having a rigid protrusion to facilitate fuel pin insertion and removal and to more positively secure the pins during reactor operation. However, it soon becomes apparent that the peripheral band surrounding the fuel assembly will therefore contain resilient and rigid protrusions, complicating the construction of the band. In addition, locating the resilient protrusions on the peripheral band necessarily results in weakening of the band. This is highly undesirable since the peripheral bands of juxtaposed fuel assemblies abut and lend lateral support to each other, and, in addition to retaining their structural integrity without damage during normal conditions, these bands must withstand impact forces generated during abnormal occurrences, for example, earthquakes. Moreover, when a reactor utilizing a grid assembly described above it utilized to power a mobile unit, such as an ice breaker ship, external vibrations may be transmitted thereto causing additional impact between the peripheral bands or between the band and its sheathing can. Hence, it is highly desirable to develop a fuel element grid plate assembly which does not utilize resilient protrusions in its peripheral band while retaining the advantages inherent in the combination resilient and rigid protrusion cells.
Furthermore, such a fuel element support assembly would offer further advantages if it could be adapted to use in a reactor that utilizes "cans" to encase each fuel assembly.